Growth of single crystal gallium nitride (GaN), aluminum nitride (AlN), and aluminum gallium nitride (AlGaN) is notoriously hard due to a lack of good “freely nucleated” seed crystals. As of currently there is no good process to produce freely nucleated III-nitrides usable for seeds or for electronic, optic, piezoelectric, and Pyroelectronic device substrates.
Fabrication of silicon carbide (SiC), GaN and AlN crystals using the sublimation method invented by Tairov and Tsvetkov in 1978 and Slack and McNelly in 1976 respectively is well known. In this method a powder sources is placed at the bottom of a closed crucible and sublimated into vapor. The vapor is transported through the empty space of the closed crucible via a temperature gradient to a seed held at a lower temperature, where the sublimated species recrystallize on a provide seed crystal.
This technique has proven sufficient for the growth of large single crystals of SiC, where diameters up to 6 inches have been readily demonstrated. But this technique has been lacking in its ability to deliver the same results for Group III-V nitride crystals, such as AlN and GaN. The fabrication of large AlN crystals with low induced internal stress of high quality has proven quite difficult. Obstacles related to available materials have been identified as a major obstacle in the growth of high-purity, large-size AlN crystals. For example, sublimation temperatures in excess of 2200° C. are required to achieve commercially viable AlN growth rates. At these temperatures, aluminum (Al) vapor is highly reactive with all but the most robust materials. A lack of large size seeds also lends to the use of thermal grain expiation via bowed thermal fields. Typically, the bowed thermal fields used to expand the size of the AlN seed crystals to produce bulk AlN induce stress into the crystals. This stress is transferred throughout the bulk of the produced AlN. Large Area SiC crystals up to 4 inches in diameter have been used to compensate for the lack of large size AlN seeds but often with disappointing results. AlN growth on SiC leads to highly order polycrystalline material where large grains are orientated in the z axis perpendicular to the SiC crystal surface but are tilted or at low-angle mismatch in the x-y plane.
Many attempts have been made to find a suitable method to adapt sublimation growth to GaN. The problem in GaN sublimation growth is due to the low transport of Gallium (Ga) vapor at the GaN sublimation temperatures. The vapor pressure of Ga over GaN at typical sublimation temperatures gives rise to nitrogen evolution without Ga evolution. Furthermore the nitrogen is far from active at these low temperatures. Active NH3 can be used to enhance the system but at a shift to higher nitrogen partial pressures. Thus usually lower temperatures are used. This in turn increases the problem with the Ga species. At lower temperatures, the surface mobility of Ga adatoms is limited and thus so are crystal growth rates. Sufficient Ga flux can be achieved by using extreme temperature gradients typically found in closed space sublimation. The extreme temperature gradients increase Ga transport and thus crystal growth speed but at the expense of crystal quality. Even with an optimized transport regime, large high quality seeds for both AlN and GaN growth have been difficult to produce. For the foregoing reasons, there is a need for a process that can produce “freely nucleated” high quality GaN AlN and Aluminum GaN crystals that can act as seeds and/or as device substrates.